While the invention may find advantageous use in other applications, it has been conceived and adopted in systems for governing the speed of multicylinder engines. In such systems, closed loop action for speed governing involves sensing and signaling the actual rotational speed of the engine output shaft, subtractively comparing the actual speed with a signaled adjustable set point speed to determine the sign and magnitude of any speed error, producing a command signal for the rate of fuel input to the engines and which changes according to a function of the speed error, and establishing or adjusting the input fuel rate according to the command signal so as to hold any speed error at or restore it essentially to zero even when abrupt and/or large changes in the engine load or the set point occur. As a general class, such a speed governing system is a closed loop control; it is preferably and commonly arranged so that corrective adjustment in the command signal and the fuel input rate are effected as "proportional-integral-derivative" (PID) function of the speed error. By providing PID responsive action to speed errors, the magnitude and durations of transient departures due to load changes can be at least approximately optimized (reduced) for the inherent transfer function (gains, phase lags and dead times) of the particular engine and load device which are being controlled. This involves tailoring or tuning the governor gain, stability and compensation factors to provide the best compromise between (i) avoidance of instability or oscillation which may result when the gain is too high, and (ii) slow or sluggish elimination of any transient speed error--which is the penalty when the gain is too low.
In speed governoring systems which utilize a programmed digital computer to create the command signal by iterated computations based upon the difference (error) between the sensed actual speed and the desired speed set point, the choice of "sampling time" or iteration rate interacts with the chosen gain of the PID function to impose alternatives or compromises that are all undesirable. If the computer iterates through its governing program to update the command signal at short repetition intervals, then the sensed speed signal will reflect torsional (or similar) instantaneous speed fluctuations and the speed error will cyclically change over the span of one revolution of the engine shaft. If the gain of corrective action is made high, then the governing controller will tell the fuel-determining actuator to change its position by significant amounts even when the average speed is essentially constant; and the fuel-rate adjusting mechanism will mechanically vibrate or oscillate to such an extent that it will literally wear itself out far short of any reasonably acceptable service life. If the gain is reduced to essentially eliminate such destructive mechanical vibrations, then the corrective response to transient errors in average shaft speed becomes intolerably slow.
On the other hand, if the computer iterates through its governing program to update the command signal at long repetition intervals, then changes in average speed will not be seen until long after they start to occur--and if the gain of the corrective action is made high to "make up" for delay, then corrective action will have phase lags which can easily result in instability and sustained osillations.
Regularly timed iterations, at a chosen and thereafter fixed rate, of a real time digital computer program to correctively adjust engine fuel input rate may be so short as to "see" the torsionally induced speed fluctuations when average engine shaft speed is in the low region of its operating range, and yet so long as to result in slow or sluggish response when the average shaft speed is in the high region of its operating range.